Have you ever had to stop in the middle of a long road trip to fix something on your ride?
Now imagine your vehicle weighs 1,500 tons, and the “road” is actually a tunnel 600 feet underground, where there is no place to pull over.
That will be the setting later this month, when workers on the Southern Nevada Water Authority’s third intake project embark on what has to be one of the most complicated and challenging maintenance jobs in the world.
The work involves the 23-foot-tall, $25 million tunnel-boring machine specially built to excavate the water authority’s new straw into Lake Mead.
A three-man crew will have to squeeze into the front of the massive digger and switch out some of the 300-pound cutter disks the machine uses to chew through solid rock. The conditions will be cramped and wet, and crew members might have to end their shifts inside a decompression chamber like deep-sea divers do.
It’s the latest technical challenge for the more than $800 million project, which involves digging a 3-mile tunnel beneath the bed of the nation’s largest reservoir to keep water flowing to the Las Vegas Valley even if Lake Mead sinks below the level of the community’s two existing straws.
The valley draws about 90 percent of its drinking water from Lake Mead.
General contractor Vegas Tunnel Constructors is scheduled to complete the third intake tunnel in the summer of 2014.
The current maintenance stop could last through the end of the year.
“Everybody’s eager to get the cutters changed and get back to tunneling,” said Erika Moonin, the water authority’s project manager for the third intake, “but they’re going to take the time to do what needs to be done.”
The tunnel-boring machine grinds its way through rock using a rotating cutter head studded with 48 ridged disks made of a special metal alloy.
Moonin said workers probably won’t have to replace all 48 disks during this stop, but they will switch out the ones with the most damage or wear.
The machine came with a second set of cutter disks for just this purpose. The disks that are removed will be refinished so they can be used again if need be, Moonin said.
To reach the disks, the three-man crew will have to squeeze through a pipe 30 inches in diameter and roughly 20 feet long to get inside the cutter head.
Before that can happen, the fractured rock around the front of the machine must be stabilized to keep water from pouring in on the workers. Otherwise, the water must be kept at bay using compressed air, creating an environment that would require the crew to limit their work time and undergo perhaps hours of decompression before returning to the surface.
Moonin said the tunnel-boring machine came equipped with its own decompression chamber, as well as a pressurized shuttle to transport workers between the chamber and the cutter head, “but going in under pressure is something they would like to avoid.”
The boring machine stopped tunneling in late September in preparation for maintenance.
For the past two months, workers have been drilling holes in the rock in front of the machine and injecting grout into the surrounding cracks in hopes of stemming the flow of water.
The machine is not yet tunneling directly beneath Lake Mead, but water from the lake is already seeping into the work area, Moonin said.
Water authority officials have called the third intake one of the most complicated and dangerous construction projects in the world, and it has already claimed a life.
On June 11, 44-year-old Thomas Albert Turner was killed in a construction accident in the tunnel 600 feet underground.
He was part of a crew installing concrete pipe segments behind the tunnel-boring machine when one of the segments slipped out of place, releasing a pressurized jet of rocks and mud that struck Turner in the head.
His death brought new scrutiny to a project that already had seen its budget grow and its schedule slip by about 20 months after a series of floods in the work area in 2010 and 2011 that forced the contractor to abandon its first tunnel and excavate a new one in a different direction.
Several more scheduled maintenance stops like this one are planned as the digger continues on its journey.
So far, the machine has advanced a little more than 1,400 feet, roughly 12 percent of the way to its ultimate goal.
Contact reporter Henry Brean at firstname.lastname@example.org or 702-383-0350.